Mg is considered one of the prospective materials for the storage of hydrogen as hydride, but unfortunately the activation of Mg requires more effort than other hydride-forming metals and the kinetics of hydriding and dehydriding is rather slow. However, alloying Mg with metals such as Ni introduces positive changes in the kinetics of hydrogen absorption. In the Mg-Ni system, the best hydrogen absorber is the intermetallic compound Mg2Ni that can be easily activated. The production of a high purity, single phase Mg2Ni, by means of mechanical alloying (MA) is possible. Previous results indicated the existence of an amorphous-like phase, or "precursor", after 12 hours of milling time. This phase partially transforms to the intermetallic compound by heating at 433 K. In the present work, the amount of Mg combined to form the precursor was quantified using a quantitative X-ray diffraction method: the internal standard method using a constant amount of Mo. The quantity of Ni combined was calculated using the magnetisation curves. Results confirmed that the precursor had a stoichiometry close to Mg2Ni intermetallic compound.